The present invention relates generally to surface acoustic wave (SAW) devices and more particularly to a SAW device having improved performance characteristics for application to RF filtering for wireless communications.
High frequency surface acoustic wave (SAW) devices are widely used in wireless communications products, particularly as radio frequency (RF) filters for transmit and receive operations. Such filters often utilize resonant SAW devices formed on single crystal piezoelectric substrates as components to generate the desired filtering function. One single crystal piezoelectric substrate, which is commonly used for RF filters, and which has some desirable characteristics for such filters, is lithium tantalate (LiTaO3). The performance characteristics of any crystal substrate vary with the selected wave propagation direction, which can be defined in terms of Euler angles. A particularly desirable cut for certain applications is described by Ueda et. al. in U.S. Pat. No. 6,037,847 and U.S. Pat. No. 5,874,869. U.S. Pat. No. 6,037,847 teaches the use of LiTaO3 with Euler angles (xcex,xcexc,xcex8) such that xcex and xcex8 fixed (set at zero), and xcexc varied depending on the metalization type and thickness used. For an electrode pattern containing Al as a primary component and forming a resonator with thickness in the range of 0.03-0.15 times a wavelength xcex9 (i.e. 3% xcex9 to 15% xcex9), the preferred rotation angle xcexc is greater than xe2x88x9251xc2x0, which corresponds to 39xc2x0-rotated YX-cut, and less than xe2x88x9244xc2x0, which corresponds to 46xc2x0-rotated YX-cut (the angle of rotation of Y-cut is determined as xcexcxe2x80x2=xcexc+90xc2x0). Additional restrictions are presented indicating that the range of Euler angles with rotational angle xcexc centered on xe2x88x9248xc2x0 (42xc2x0-rotated YX-cut) is preferred. For electrode patterns having Cu as a primary component, with electrode thickness of 0.9% xcex9 to 4.5% xcex9, a rotational angle xcexc greater than xe2x88x9251xc2x0 but less than xe2x88x9244xc2x0 is selected. For electrode patterns containing Au as a primary component and having thickness in the range of 0.4% xcex9 to 2.1% xcex9, a rotational angle xcexc greater than xe2x88x9251xc2x0 but less than xe2x88x9244xc2x0 is selected. As a result, Ueda ""847 uses a rotational angle xcexc in ranges greater than xe2x88x9251xc2x0 but less than xe2x88x9244xc2x0. U.S. Pat. No. 5,874,869 teaches the use of LiTaO3 with Euler Angles xcex and xcex8 fixed (nominally zero), and xcexc in a range between xe2x88x9250xc2x0 and xe2x88x9248xc2x0 for multi-mode SAW devices with a range of specific device design characteristics.
While the Ueda ""847 and ""869 patents do not specifically state values for Euler angles xcex and xcex8, the description of piezoelectric substrate having an orientation rotated about an X-axis thereof, from a Y-axis thereof, toward a Z-axis thereof, with a rotational angle in a specified range, and the direction of propagation of the surface acoustic wave set in the X-direction would lead one skilled in the art to appreciate that the first Euler angle xcex and the third Euler angle xcex8 are equal to zero.
SAW devices built on the aforementioned orientations of LiTaO3 utilize leaky surface acoustic waves (LSAW). A leaky wave has higher propagation velocity, as compared to SAW, which is an advantageous feature for high-frequency SAW devices. Though normally a leaky wave propagates along crystal surface with non-zero attenuation, caused by radiation of bulk acoustic waves into the bulk of crystal, under certain conditions this attenuation tends to zero. One class of leaky waves having negligible attenuation is quasi-bulk waves. With a free surface of a crystal, the mechanical boundary condition can be satisfied for a bulk wave propagating along the boundary plane and polarized in this plane, thus called horizontally polarized wave. In any crystal, orientations in which one of bulk waves satisfies mechanical boundary conditions, form lines in crystallographic space defined by three Euler angles. For LiTaO3, such orientations were previously discussed in a publication by N. F. Naumenko, Sov. Phys.-Crystallography 37, pp. 220-223, 1992. In particular, it was found that one of these orientations is known as 36xc2x0-rotated YX-cut, Euler angles (0xc2x0, xe2x88x9254xc2x0, 0xc2x0). This is a symmetric orientation characterized by the propagation direction parallel to X axis and a normal to the boundary plane lying in the plane of reflection symmetry YZ of LiTaO3. The fast shear bulk wave propagating along X-axis and polarized in the plane of 36xc2x0-rotated YX-cut is strongly piezoelectrically coupled with the electric field component along X-axis, due to proximity of the corresponding effective piezoelectric module to its absolute maximum for LiTaO3. As to the promising characteristics of 36xc2x0-rotated YX-cut for application in SAW devices, reference should be made to K. Nakamura et al., Proc. 1977 IEEE Ultrasonics Symposium, pp. 819-822.
Electrical boundary conditions change the nature of the bulk wave and make it quasi-bulk with propagation velocity slightly lower than that of the bulk wave. The effect of mass loading and electric boundary conditions in an electrode pattern disposed on the surface of 36xc2x0-rotated YX cut results in increasing attenuation or propagation loss. However, as described in U.S. Pat. No. 6,037,847 to Ueda et al., orientation with nearly zero propagation loss does not disappear but continuously moves from 36xc2x0YX to 42xc2x0YX cut while Al electrode thickness increases from zero to 0.08xcex9. Similarly, orientations with zero LSAW attenuation were found for electrode patterns containing Cu or Au as a primary component, as functions of metal thickness. According to the detailed description of a method used for evaluation of propagation loss due to scattering of LSAW into slow shear bulk waves, reported by Hashimoto (K. Hashimoto et al., Proc. 1997 IEEE Ultrasonics Symposium, pp. 245-254), minimum propagation loss at the lower edge of a stopband of Bragg""s reflection, which corresponds to the resonant frequency of LSAW resonator, was chosen as a criterion of optimizing cut angle. However, propagation loss is a function of frequency. Thus, it is desirable to minimize its average value in a bandwidth. As will be seen, the present invention minimizes propagation loss simultaneously at resonant (fr) and anti-resonant (fa) frequencies.
To explain the effect of propagation loss on a filter performance, reference is now made to FIG. 1, which is an example of a ladder filter, comprising three shunt (R4,R5, R6) and three series (R1, R2, R3) resonant SAW structures and utilizing 42xc2x0-rotated YX-Cut LiTaO3 substrate. For the devices under consideration, resonant SAW structures are used as both series and as parallel (shunt) components within a composite device structure, which may include lattice-like regions. In ladder filters it is common to have the anti-resonant frequency of the shunt elements approximately equal to the resonant frequency of the series elements. The lower passband edge of a filter is then determined by propagation loss at the resonant frequency of the shunt elements and the upper passband edge is determined by the propagation loss at the anti-resonance of the series elements. Thus, the propagation loss at both frequencies, resonant and anti-resonant one, are significant and it is desirable that they be simultaneously minimized.
FIG. 2 shows propagation loss at resonant and anti-resonant frequencies calculated for 42xc2x0-rotated YX cut LiTaO3 with Al as electrode material, as functions of electrode thickness normalized to LSAW wavelength, h/xcex9. These and other calculations were made with material constants of LiTaO3 reported by Taziev (R. M. Taziev et al., Proc. 1994 IEEE Ultrasonics Symposium, pp.415-419), though it was found that the results do not change significantly if another set of material constants is used, for example, the constants reported by Kovacs (G. Kovacs et al. Proc. 1990 IEEE Ultrasonics Symposium, pp.435-438).
By way of example, let an electrode thickness of 10% xcex9. In recent RF filters, especially for GHz applications, such electrode thickness is rather conventional due to high operating frequencies and hence short wavelengths. While propagation loss at resonant frequency is fairly low, about 0.003 dB/xcex9, at anti-resonance it is about 0.03 dB/xcex9, which is 10 times higher. As a result, a frequency response of a filter is expected to be non-symmetric, with larger degradation of a high-frequency passband edge, and increased shape factor. FIG. 3 illustrates the effect of propagation loss at resonance and anti-resonance on SAW filter performance and demonstrates that if propagation loss is minimized at average frequency fo=(fr+fa)/2, lower insertion loss and better shape factor can be provided, as compared to the cases when propagation loss is minimized either at resonant or at anti-resonant frequency. A more desirable shape factor is expected due to wider bandwidth and steeper edges of the passband.
In view of the aforementioned, one purpose of the present invention is to provide improved performance, and, in particular, to reduce insertion loss and improve shape factor, in SAW filters comprising resonator-type elements, using selected orientations of LiTaO3 with simultaneously optimized propagation loss at resonant and anti-resonant frequencies, while the electrode thickness varies in a wide range from 1% xcex9 to 15% xcex9.
In particular, there is a strong need to provide substrate cuts with fairly low propagation loss (desirably less than 0.01 dB/xcex9) in the interval of thicknesses from 8% xcex9 to 15% xcex9, for an electrode pattern with Al as a primary component. According to FIG. 13 of U.S. Pat. No. 6,037,847, if an electrode thickness exceeds 8% xcex9, no LiTaO3 orientation in the interval from 36xc2x0-YX to 46xc2x0-YX can provide as low a propagation loss as it is in a 42xc2x0YX cut with Al thickness 7.5% xcex9. For example, with 10% Al thickness, minimum propagation loss was found to be about 0.01 dB/xcex9. Further, there is a need for substrate cuts with optimized propagation loss when Au is utilized as a primary component of electrode material, with electrode thickness in the range from 1.5% xcex9 to 2.5% xcex9 and with Cu utilized as a primary component of electrode material, when electrode thickness is in the range from 3% xcex9 to 6% xcex9.
A variety of specified values of electrical parameters in RF filters for different applications requires piezoelectric substrates with different values of LSAW characteristics, in particular, different electromechanical coupling coefficients. However, the requirement of low insertion loss and high operating frequencies restricts the substrate cuts, which are commonly used in RF filters, to xcexcxe2x80x2-rotated YX-cuts of LiNbO3 and xcexcxe2x80x2-rotated YX-cuts of LiTaO3, with rotation angle xcexcxe2x80x2 selected according to the required thickness of electrodes. On the other hand, a variety of substrate cuts with fairly low propagation loss can be provided due to non-symmetric orientations of LiTaO3, defined by Euler angles (xcex, xcexc, xcex8) (with xcex and xcex8 being nonzero). By the example of 36xc2x0-42xc2x0 rotated Y-cuts of LiTaO3, it was demonstrated (U.S. Pat. No. 6,037,847) that low-attenuated leaky waves of quasi-bulk nature continuously move in crystallographic space with increasing electrode thickness, rather than disappear.
The teachings of the present invention will show that such behavior is also typical for non-symmetric orientations in which the fast shear surface-skimming bulk wave (SSBW) satisfies mechanical boundary condition on a free surface. Moreover, non-symmetric orientations with optimized propagation loss form a continuous line in crystallographic space, and this line crosses a symmetric point (0, xcexc, 0). Therefore, locating this line can provide for adjusting the propagation direction with occasional deviation of a crystal cut plane from a symmetric orientation, in order to retain low propagation loss.
In view of the foregoing background, it is therefore an object of the present invention to provide a piezoelectric substrate with an optimum orientation for use in high frequency (RF) SAW devices, which can eliminate known disadvantages of the prior art substrate orientations.
Another object of the present invention is to provide a SAW device, comprising resonator-type elements, with improved performance using orientations of LiTaO3 with simultaneously optimized propagation loss at resonant and anti-resonant frequencies, while electrode thickness varies in a wide range from 1% xcex9 to 15% xcex9, where xcex9 is acoustic wavelength.
Another object of the present invention is to provide a variety of electrical parameters in SAW devices for RF applications using non-symmetric orientations defined by Euler angles (xcex, xcexc, xcex8) (with xcex and xcex8 being nonzero) and having propagation loss less than 0.01 dB/xcex9 and electromechanical coupling factor greater than 0.07, while electrode thickness of the pattern is larger than 1% xcex9 and less than 15% xcex9.
Another object of the present invention is to provide for adjusting the propagation direction with occasional deviation of a crystal cut plane from a desired symmetric orientation (0, xcexc, 0), to retain low propagation loss. This is achieved by finding such relationship between Euler angles xcex and xcex8, which describes orientations (xcex, xcexc, xcex8) with optimized propagation loss, while the angle xcex varies from xe2x88x924xc2x0 to 4xc2x0 and angle xcexc is fixed.
Another object of the present invention is to provide a SAW device comprising a piezoelectric substrate of a single crystal LiTaO3 with an electrode pattern disposed on a surface of said piezoelectric substrate and forming resonator, wherein a thickness of the electrode pattern is in the range from 1% to 15% xcex9 and Al is used as a primary component of electrode material, and wherein a piezoelectric substrate has orientation defined by the Euler angles (xcex, xcexc, xcex8), with angle xcex in the range from xe2x88x9240xc2x0 to +40xc2x0, angle=xcexc in the range from xe2x88x9252xc2x0 to xe2x88x9236xc2x0, and angle xcex8 in the range from (xe2x88x921.365*xcexxe2x88x924)xc2x0 to (xe2x88x921.365*xcex+4)xc2x0, wherein either angles xcex or xcex8 are not equal to zero.
Another object of the present invention is to provide a SAW device comprising a piezoelectric substrate of a single crystal LiTaO3 with an electrode pattern disposed on a surface of the piezoelectric substrate and forming resonator, wherein thickness of a said electrode pattern is in the range from 1% to 2.5% xcex9 and Au is used as a primary component of electrode material, and wherein a piezoelectric substrate has orientation defined by the Euler angles (xcex, xcexc, xcex8), with angle xcex in the range from xe2x88x924xc2x0 to +4xc2x0, angle xcexc in the range from xe2x88x9252xc2x0 to xe2x88x9236xc2x0, and angle xcex8 in the range from (xe2x88x921.365*xcexxe2x88x924)xc2x0 to (xe2x88x921.365*xcex+4)xc2x0, wherein either angles xcex or xcex8 are not equal to zero.
Another object of the present invention is to provide a SAW device comprising a piezoelectric substrate of a single crystal LiTaO3 with an electrode pattern disposed on a surface of the piezoelectric substrate and forming a resonator, wherein a thickness of the electrode pattern is in the range from 1% to 6% xcex9 and Cu is used as a primary component of electrode material, and wherein a piezoelectric substrate has orientation defined by the Euler angles (xcex, xcexc, xcex8), with angle xcex in the range from xe2x88x924xc2x0 to +4xc2x0, angle xcexc in the range from xe2x88x9252xc2x0 to xe2x88x9236xc2x0, and angle xcex8 in the range from (xe2x88x921.365*xcexxe2x88x924)xc2x0 to (xe2x88x921.365*xcex+4)xc2x0, wherein either angles xcex or xcex8 are not equal to zero.
Another object of the present invention is to provide a SAW device comprising a piezoelectric substrate of a single crystal LiTaO3 with an electrode pattern disposed on a surface of the piezoelectric substrate and forming a resonator, wherein a thickness of the electrode pattern is in the range from 5% to 15% xcex9 and Al is used as a primary component of the electrode material, and wherein a piezoelectric substrate has orientation defined by the Euler angles (0, xcexc, 0), with angle xcexc in the range from xe2x88x9244xc2x0 to xe2x88x9236xc2x0.
Another object of the present invention is to provide a SAW device comprising a piezoelectric substrate of a single crystal LiTaO3 with an electrode pattern disposed on a surface of the piezoelectric substrate and forming a resonator, and wherein a thickness of the electrode pattern is in the range from 1.5% to 2.5% xcex9 and Au is used as a primary component of electrode material, and wherein a piezoelectric substrate has orientation defined by the Euler angles (0, xcexc, 0), with angle xcexc in the range from xe2x88x9244xc2x0 to xe2x88x9236xc2x0.
Another object of the present invention is to provide a SAW device comprising a piezoelectric substrate of a single crystal LiTaO3 with an electrode pattern disposed on a surface of the piezoelectric substrate and forming a resonator, wherein a thickness of the electrode pattern is in the range from 3% to 6% xcex9 and Cu is used as a primary component of the electrode material, and wherein a piezoelectric substrate has orientation defined by the Euler angles (0, xcexc, 0), with angle xcexc in the range from xe2x88x9244xc2x0 to xe2x88x9236xc2x0.